This invention relates to a process and an apparatus for in-situ bioremediation of a chemical contaminant in an aquifer. The invention further relates to a method of delivery bacterial culture in a subsurface, and an oxygen delivery system for the remediation of aquifers contaminated with a chemical compound such as methyl-t-butyl ether (MTBE).
The 1990 Clean Air Act Amendments mandated that gasoline suppliers reformulate their gasoline blends to be cleaner burning, and to have less impact to the atmosphere. In response to this mandate, gasoline suppliers began to blend their fuels with oxygenate chemicals, such as alkyl ethers. In particular, methyl-tert-butyl ether (hereinafter xe2x80x9cMTBExe2x80x9d), was used quite extensively, and often comprised up to 10 to 15% by volume of unleaded gasoline.
Now, having used oxygenated fuels for more than a decade, it has become clear that these cleaner-burning fuels pose great threats to groundwater resources. In particular, many oxygenate chemicals are very soluble in water and are slow to degrade in the environment; hence they tend to accumulate in water resources once released to the environment. For example, MTBE has been detected in groundwater with high frequency in many states and there are well-documented cases of impacts to municipal water supply wells. In some cases these impacts result from accidental gasoline spills; in other cases they are attributed to the re-deposition of chemicals emitted to the atmosphere from partially combusted automobile exhaust.
It is also now known that when oxygenate chemicals including alkyl ethers, such as MTBE and tertiary butyl alcohol (hereinafter xe2x80x9cTBAxe2x80x9d), are found in the subsurface, then they are resistant to biodegradation under natural conditions. This is the main reason for their persistence and accumulation in soil and groundwater.
With the increase in our knowledge of the behavior of these chemicals, and the increase in documented impacts, regulatory agencies are now vigorously enforcing cleanup standards for MTBE and TBA in groundwater. As a result, practitioners are searching for technologies that can clean-up soil and aquifers contaminated with oxygenates such as MTBE and TBA.
Shallow contaminated soil can be treated by excavating the contaminated soil and then treating it above-ground. However, in most cases, it is preferable to treat contaminated soils in-situ so as to minimize disturbance of the site and prevent further release of the contaminants to the atmosphere.
U.S. Pat. Nos. 5,750,364, 5,811,010, 5,902,734, and 6,238,906 assigned to Shell Oil Company, relate to utilizing mixed or single cell bacterial cultures for aerobically degrading alkylethers and TBA to non-toxic carbon dioxide and water. U.S. Pat. No. 6,503,395, assigned to Shell Oil Company, relates to the in-situ purification of ethers and/or alcohols. U.S. Pat. No. 6,503,395, assigned to Shell Oil Company, discloses a method of delivering oxygen the subsurface. However, in-situ remediation of sub-surface chemical contaminants by delivering contaminant-degrading aerobic bacteria to the sub-surface (this overall process will hereinafter be referred to as xe2x80x9cbio-augmentationxe2x80x9d) has not historically been embraced by the practice. It is acknowledged that delivering and maintaining non-indigenous microorganism cultures in the subsurface is a very difficult task. Moreover, there is a need for an effective and economical method which would deliver optimal level of oxygen-containing gas to grow and maintain aerobic microorganisms as well as to supply oxygen required for bioremediation.
In operations for injecting biomass such as microorganism cultures in the subsurface for remediation, injection has typically been accomplished by using a hollow-core drill rod, such as a 1 to 2 inch diameter drill rod, with a disposable tip. The drill pipe is pushed into the soil using a direct-push system. In an attempt to avoid clogging, the drill pipe is pushed to the desired depth with a disposable tip, the drill pipe is withdrawn upward a few inches with the tip left in place. Then, injection of the biomass is begun, e.g. at a rate of approximately 5-20 gallons/minute, followed by withdrawal of the pipe upwards, e.g. from about 2 ft. to about 10 ft., and then a repeat of the injection. The operation is continued over the vertical zone to be treated.
A common occurrence is that, when the flow of biomass is discontinued, in order to pull the drill string upward or add more drill string or change connections, there is a backflow of fluidized soil and water into the injection ports and hollow drill pipe. This backflow clogs the injection ports and the pipe with soil particles. At this point, restarting the injection is impossible without withdrawing the drill string, clearing the soil plug or replacing the pipe, and starting over. In the field, slurry injection operations often suffer significant downtime, restricted operations, and failure due to this backflow of soil. Presently, there does not seem to be any tool available in technology relating to injecting biomass which addresses this problem.
In addition, there is a need for a system that permits the operator to inject every few feet starting, for example at the top of a water table and moving downward, or to move up and down. The current technology requires injection only as the drill pipe is withdrawn. A typical patented direct-push technology for environmental soil sampling is called Geoprobe(trademark) and there are a number of commercially available sampling and injection systems available that use that technology.
Now, with the need to treat aquifers and soils contaminated with more recalcitrant chemicals, there is a need for a bio-augmentation process which can successfully deliver and maintain non-indigenous aerobic microorganism cultures which requires oxygen for growth and biodegradation, in the subsurface. More specifically, there is a need for an effective bio-augmentation process for remediating oxygenate chemicals such as alkyl ethers, particularly MTBE, and TBA contamination in soils and groundwater.
Now, with the need to treat aquifers and soils contaminated with more recalcitrant chemicals, there is a need for a bio-augmentation process which can successfully deliver and maintain non-indigenous microorganism cultures in the subsurface. More specifically, there is a need for an effective bio-augmentation process which effectively delivers oxygen and microorganisms for remediating oxygenate chemicals such as alkyl ethers, particularly MTBE, and TBA contamination in soils and groundwater.
This invention relates to a method and apparatus for the in-situ bioremediation of aquifers contaminated with chemical(s), particularly oxygenate chemicals such as MTBE, and/or t-butyl alcohol (TBA) by injecting into the aquifers a microbial culture that degrades said chemical(s) such as MTBE and/or t-butyl alcohol. In particular, the invention uses (a) a bacterial culture capable of aerobically degrading the target chemicals, (b) an apparatus for continuously or intermittently delivering bacterial culture to subsurface without backflow of soil and other materials into the injection pipe while not pumping or while making changes to the drill string, pump and hose attachments to the drill string, and (c) an oxygen delivery system injecting, by means of a network of at least two conduits which extend in and/or below the treatment zone, an oxygen-containing gas at a pressure of at least 5 psig (pounds per square inch gauge) above the hydrostatic pressure at each point of delivery, by pulsed injection, at a frequency in the range of from about once per week to about 10 times per day optimized so as to maximize aerobic biodegradation while maintaining less than 50%, preferably less than 10% volatilization of contaminants.
To reach the optimal delivery of the oxygen-containing gas, the injection frequency and volume at each injection point are adjusted to have the relationship according to the following equation:
e[(xe2x88x92Vxc3x97Fxc3x97Nxc3x97H)/(Wxc3x97Bxc3x97Q)] greater than 0.50 (preferably greater than 0.80, more preferably greater than 0.90, still more preferably greater than 0.93)
Wherein:
e=natural exponential
V=volume of gas injected at each injection point (ft3)
F=frequency of injections (number of injections per day)
N=number of gas injection points
W=width of treatment zone perpendicular to groundwater flow path (ft)
B=vertical thickness of treatment zone (ft)
Q=specific discharge of groundwater to treatment zone (ft/day)
H=Henry""s Constant for contaminant of interest ((mg/L-water)/(mg/L-air))